At the time of purifying silicon used for a solar cell, oxidative purification is performed to remove boron (B) or contained impurities through heating by plasma arc or plasma gas (see, for example, Patent Document 1 and Non-patent Document 1). In general, silicon purification devices used in the silicon purification are categorized into two types, that is, a transferred type and a non-transferred type, depending on arrangement of the electrodes for generating the plasma arc.
The transferred type device includes a cathode electrode in a plasma torch. Only a low direct current is applied to a nozzle port of the plasma torch. An electrically conductive member provided at a bottom portion of a crucible is used as an anode electrode, and the plasma arc is generated by applying the direct current through both of the electrodes. The plasma arc is injected from the nozzle port toward the silicon metal loaded in the crucible, thereby heating the silicon metal (see, for example, Patent Document 1).
On the other hand, the non-transferred-type device includes the cathode electrode and the anode electrode in the plasma torch. The plasma arc is generated by applying a direct current through both electrodes in the plasma torch. The generated plasma arc is injected from the nozzle port, which also serves as the anode electrode, of the plasma torch toward the silicon metal loaded in the crucible, thereby heating the silicon metal (see, for example, Patent Documents 1 and 2).
In comparison between the transferred type and the non-transferred type, the plasma arc directly hits the silicon metal subjected to the heating in the transferred type, and hence, the transferred type exhibits excellent heating efficiency as compared with the non-transferred type.
In the non-transferred type, the silicon metal subjected to the heating is hit only by the plasma gas. In this case, the pinch effect, which can be seen in the transferred type, does not work and the plasma gas is not focused, so that the plasma gas after hitting the surface of the silicon melt tends to expand. It is considered that this leads to an increase in the contacting area between the plasma gas and the silicon melt surface, and increases the speed of removing the boron in the oxidative purification for removal of the boron and the like as compared with the transferred type, whereby excellent efficiency in silicon purification can be obtained.